In present day fiberoptic communication fields, and in particular within the telecommunication field, it is desirable to be able to tap light signals in order to ascertain the traffic status of the optical fiber. At present, the light signals are tapped on the fiber with the aid of a permanently attached tapping device. The fiber is comprised of a light conducting core and a cladding. In order to make the light signal accessible to the tapping device, either the cladding is removed at the tapping site or the fiber is bent. The tapping device functions to tap the light signal from the core through its evanescence field.
U.S. Pat. No. 3,982,123 discloses two methods of tapping a light signal from an optical fiber without requiring the fiber to be broken. The concept of this patent is to look into the fiber so as to ascertain its traffic status, and signal tapping can be effected anywhere whatsoever without disturbing the traffic. This is achieved by placing the tapping device, which in this case is comprised of a material which incorporates a photodetector, on a light conducting core or on the fiber so that tapping of light signals can be effected. The optical fiber is comprised of a core that has low optical losses and cladding which has a lower refractive index than the core.
A first method described in the patent involves removing all, or practically all of the cladding material from the fiber. The detector is then placed securely on the light conducting core, the stripped region of which must be at least three times the wavelength in the optical fiber.
Another method of tapping light signals is to bend the optical fiber without removing the cladding material. This enables the light signals to be extracted through the cladding and captured by a photodetector. Tapping is effected permanently in both cases.
U.S. Pat. No. 4,784,452 describes a method in which tapping is effected with the aid of a tapping device placed on an optical fiber. This fiber is comprised of a light conducting core and at least one cladding material. The tapping device, a probe, is an optical fiber of the same type as the fiber from which the signals are tapped. This probe has a free end which includes a light conducting core. In order to tap light signals from the fiber, it is necessary to remove the cladding so as to expose the core. The probe is used at this exposed region, with the free end of the probe placed against the bared part of the fiber. In order to obtain the best possible tapping effect, it is necessary to adapt the angle defined by the probe axis and the fiber axis. A coupling medium connects the region at the probe and the bared part of the fiber and conducts light signals from the bared part of the fiber to the probe. The coupling medium, which is a solid and hard material, fixes the probe in relation to the fiber.
Various experiments have shown that the light conducting core may be comprised of polyimide. In the article "Dependence of Precursor Chemistry and Curing Conditions on Optical Loss Characteristics of Polyimide Wageguides" by C. P. Chien and K. K. Chakravorty at Boeing Aerospace and Electronics, Seattle, USA, SPIE vol 1323, Optical Thin Films III, New developments (1990), it is disclosed that polyimide is a good material for the optical fiber core. Polyimide has good thermal stability and a dielectric index of 3.5, which is compatible with other IC-materials. The material functions well as a light transmitter, such as in optoelectric circuits in GHz frequency range. The advantage of polyimide is that when manufacturing cores, the cores can be packed tightly together. Additional polyimide data is that it has a refractive index of 1.6 (1.58-1.62) and optical losses in the core of about 1 dB/cm when exposed to ultraviolet light.
Experiments have been carried out with a silicone elastomer as an index matching medium for the light conductive core. The article "Index Matching Elastomers for Fiber Optics" by Robert W. Filas, B. H. Johnson and C. P. Wong at AT&T Bell laboratories, N.J. USA in the magazine IEEE, Proc. Electron. Compon. Cont., 39th, 486-9, disclose that silicone elastomers are good core index matching materials. Copolymer reflection as a function of the diphenyl concentration and temperature is obtained by measuring the reflection strength of a single mode waveguide whose core has been encapsulated in an elastomer. It is possible to obtain the same refractive index on a silicone rubber material as the refractive index of the core. The silicone rubber can be used as an interface between different components. Another method is to use the silicone rubber as protection against moisture and dust, for instance.
At present, air is used as the refractive medium to the light conducting core of the lightwave conductor. Air has a much lower refractive index than polyimide. The refractive index of air is 1, whereas the refractive index of the polyimide is 1.6 and the refractive index of the silicone rubber is 1.5.
One drawback with the earlier known solutions is that light signals are tapped from optical fibers with the aid of permanently attached devices. This means that light signals are tapped from the fiber at a specific place thereon at which cladding has been removed. The earlier solutions are encumbered with a number of additional drawbacks. One of these drawbacks is that light signals can only be tapped on fiber waveguides and that it is necessary to remove cladding from the place at which tapping shall take place. The tapping device must be placed firmly on the optical fiber at that place from which the cladding has been removed. Tapping in permanent branches results in excessively high losses.